CEE Kerala Engineering CEE Kerala Engineering Solved Paper-2003

  • question_answer
    For\[|x|<1,\]let\[y=1+x+{{x}^{2}}+.....\]to\[\infty ,\]then\[\frac{dy}{dx}-y\]is equal to:

    A)  \[\frac{x}{y}\]                                  

    B)  \[\frac{{{x}^{2}}}{{{y}^{2}}}\]

    C)  \[\frac{x}{{{y}^{2}}}\]                   

    D)         \[x{{y}^{2}}\]

    E)  \[\frac{x-1}{{{y}^{2}}}\]

    Correct Answer: C

    Solution :

    \[\because \]     \[y=1+x+{{x}^{2}}+....\infty \] \[\therefore \] \[y=\frac{1}{1-x}={{(1-x)}^{-1}}\] \[\Rightarrow \]               \[\frac{dy}{dx}=-\frac{1}{{{(1-x)}^{2}}}(-1)=\frac{1}{{{(1-x)}^{2}}}\] \[\therefore \]  \[\frac{dy}{dx}-y=\frac{1}{{{(1-x)}^{2}}}-\frac{1}{1-x}=\frac{1-1+x}{{{(1-x)}^{2}}}\]                 \[=\frac{x}{{{(1-x)}^{2}}}\] \[\Rightarrow \]               \[\frac{dy}{dx}-y=\frac{x}{{{y}^{2}}}\]   


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